REVIEWED BY

Alicia Spittle PhD
Senior Research Fellow
Murdoch Children’s Research Institute, Royal Women’s Hospital & the University of Melbourne

TYPE OF INVESTIGATION

Systematic review

QUESTION

(1) What language exposure do preterm infants in the neonatal unit receive?; (2) What is the optimal level of language exposure to optimize neurodevelopmental outcomes in preterm infants in the neonatal unit?

METHODS

Design: Systematic review

Search terms: NICU OR (neonat* OR infant OR preterm OR prem* OR newborn OR baby OR babies) AND (“intensive care” OR “critical care” OR “special care” OR “nursery”) AND (language OR lingual OR verbal OR speech OR spoken OR read* OR auditory OR sound)

Exclusion criteria: studies which were: not original intervention or observational studies; not inclusive of preterm infants (<37 weeks); did not measure language exposure or use language as an intervention; took place while the infant was not an inpatient of the neonatal unit.

MAIN RESULTS:

The authors identified ten articles that were eligible for inclusion in their study – four documenting observational research, and six documenting intervention research (including any study design, e.g. case-control, interrupted time series, randomized controlled trial). The authors then tabulated the included observational and intentional studies, and used the Effective Public Health Practice Project Qualitative Assessment Tool for Quantitative Studies to evaluate the methodological rigor of included studies. Methodological ratings of the included studies varied from weak to strong. The populations studied also varied widely; for example mean gestational age at birth for participants (where reported) ranged from 25.7 weeks to >36 weeks, and mean birthweight ranged from 896 to 2410 grams.

The outcomes of observational studies were described under the headings ‘Adult Word Count,’ ‘Infant Vocalisations and Conversation Turns’, and ‘Decibels.’ Four studies reported adult word count (AWC) or AWC per hour. AWC was generally low across studies. Increases in AWC were mostly positively correlated with parental presence and gestational age; although one study found a decline in AWC at 36 weeks for infants with visible signs of medical complexity (respiratory cannula, incubator). An increase in infant vocalisations between 32 and 36 weeks was noted in one study, seemingly correlated with parent presence, and exposure to more adult words. Higher-than-recommended ambient sound levels (i.e. >45dBA average and >65bdA peak) were seen in two of the included studies.

Examined cardiorespiratory outcomes included oxygen saturation (SpO2) (five studies), heart rate (four studies), cardiorespiratory events (three studies), respiratory rate (one study), and days of respiratory support (one study). Effects on both oxygen saturation and heart rate were inconsistent across studies, but suggested possible minor decreases in each metric with exposure to language. There were significant concerns around confounding (i.e. the effects of uncontrolled variables that may interact with both the independent and dependent variables, and thus reduce internal validity) with regards to effects on cardiorespiratory events, and so no conclusions could be drawn. Respiratory rate was lower in the exposure group following intervention in the one study that examined this outcome; and there was no difference in days requiring respiratory support.

Outcomes categorised as neurobehavioural included pain scores and behavioural state. One study found no significant effect on pain score. Neurobehavioural outcomes were inconclusive, and no definitive conclusions could be drawn.

Neuromotor outcomes were reported across three studies: two examining movement-related outcomes of language intervention, and one reporting effects on brain development. Possible effects of language intervention on general movement scores, upper limb dominance, and auditory cortex thickness were found.

Two studies examined nutritional outcomes of language exposure interventions, and results were equivocal: no differences were found in fluid intake, days to regain birth-weight, or days of nil oral intake; the included studies differed on whether weight gain or days to full enteral feeds were significantly different between groups; and one study found a significant decrease in feed intolerance in the treatment group.

The one included study that examined parent-infant outcomes of language intervention (a book-reading intervention) determined that participating parents enjoyed reading to their infants, felt it promoted intimacy and normalcy, and continued to read regularly to their infants post-discharge.

CONCLUSION:

The authors conclude that, based on the extent literature, hospitalised preterm infants are exposed to comparatively low levels of language, but to higher than recommended levels of sound. More language exposure was noted for those infants with increased parental presence, and as infants’ gestational age increased. Correspondingly, older infants also vocalized more.

Intervention studies were highly heterogeneous, and their outcomes were equivocal and often contradictory. There was some suggestion overall of short-term beneficial outcomes of language interventions, but the authors point out that there was a high chance of confounding. The authors also suggest that there may be safety implications of interventions which provide input stimulus above the recommended level of 45dBA average and 65dBA peak. To address gaps in the literature, they recommended large clinical trials examining both short-term clinical and physiological outcomes, and longer-term developmental outcomes up to at least two years of age; and further research into typical, in-utero language exposure.

COMMENTARY:

Hospitalised infants have long been known to be physiologically and neurodevelopmentally vulnerable, yet hypothesised risk factors seem to contribute only part of the puzzle (1). Recently there has been an upsurge in research and commentary questioning whether language exposure during an infant’s hospital stay might constitute a significant risk factor for poor outcomes – and thus fertile ground for early intervention.

In this systematic review, Best, Bogossian, and New (2) examined observational and intervention studies of language exposure for hospitalized preterm neonates. Ten studies met their inclusion criteria. The authors found evidence that hospitalized preterm infants are exposed, simultaneously, to high levels of noise and low levels of language. Further, intervention studies, though reporting some promising outcomes in the short term, have been limited by heterogeneity and risk of confounding. The timeliness of this review is demonstrated by the contemporaneous publication of more intervention studies in the area of language exposure in the NICU (3,4), and similarly targeted systematic reviews. (5,6)

The authors reflect that the literature is currently limited, given factors mentioned above. Correspondingly, language intervention (as a component of neurodevelopmental care) and speech language pathology services are not standard in neonatal hospital contexts (7,8). Yet if the growing number of publications is any indication, interest is increasing, and a more robust evidence base will soon emerge. Adequately powered, randomized controlled trials using appropriate stimuli will be critical, and, as the authors suggest, longer term follow up is indicated. The authors suggest “2 years’ corrected age and beyond” (2) as a target for longitudinal studies of intervention outcomes. We emphasise “and beyond”, given that language development is notoriously unstable at two years’, and longer-term follow up may be necessary to obtain valid outcomes. (9)

While awaiting the outcomes of such studies, clinicians may elect to trial strategies that increase language exposure for hospitalized preterm infants, given that the hospital language environment is considerably more impoverished than the in-utero language environment at equivalent gestational age, and that the literature seems to suggest some benefit (2,5,6). Language exposure is pertinent to everyone who interacts with hospitalized infants, but speech language pathologists and music therapists are particularly well suited to promote optimal input, and to train parents and staff in how to increase language exposure safely and appropriately. Increasing metrics such as Adult Word Count could be as simple as adopting practices that are already promoted to parents of infants in the community: reading and singing to babies, narrating care tasks etc. Importantly, the infant’s response to language exposure should be individually targeted, ensuring the sound level is appropriate, and the infant maintains physiological and neurobehavioural stability.

The sensory deprivation experienced by hospitalized preterm infants is increasingly perceived as problematic. Paucity of language exposure is a critical component of this deprivation, but further investigation will be required to determine the relationship between language deprivation and relevant outcomes (especially language development); and to explore whether simple language- and music-based interventions are effective in protecting preterm infants from the effects of an extra-uterine environment.

REFERENCES

Saigal S, Doyle LW. An overview of mortality and sequelae of preterm birth from infancy to adulthood. The Lancet 2008; 371:261–269.